Problem: In physics class, the girl to boy ratio is $8$ to $3$. If there are a total of $33$ students, how many girls are there?
A ratio of $8$ girls to $3$ boys means that a set of $11$ students will have $8$ girls and $3$ boys. A class of $33$ students has $3$ sets of $11$ students. Because we know that there are $8$ girls in each set of $11$ students, the class must have $3$ groups of $8$ girls each. There is a total of $24$ girls in physics class.